There is a whole "club" of people over on Calguns who have done exactly this. They have cut, then re-welded the receivers to make an Obrez, they have shot them, had a lot of fun with them and if this flies in anti-gun California, I think that legally it will fly anywhere as a regular Title I firearm.

As long as you had proof that this is what you did, wouldn't it be the same as buying a cut receiver Bren and reassembling into a semi-auto Title I firearm? There are several companies that do this.